Blown polymeric film structures used, for example, in the packaging arts, are formed by extruding or coextruding continuous single-layer to seven-layer polymeric films through a blown film extruder in a form known in the art as a "bubble." The extruding process generally includes the introduction of a flow of molten polymer to an extruder head, the polymer flow then being directed through the head under controlled heat and pressure conditions to an adjustable annular or other shape die gap. In some types of blown film apparatus, die oscillation systems evenly distribute the molten polymer at the extruder head. The emerging polymer begins to solidify into a tubular structure or bubble which is cooled by a pressurized air flow from a cooling ring adjacent to the extruder head, followed by longitudinal haul-off of the bubble in the direction of the extrusion output. Generally, such haul-off followed by wind-up of the film onto a roll is directed in a vertically upward direction so as to compensate for or eliminate gravitational effects on the film as it solidifies prior to reaching the wind-up apparatus.
An important problem in the art to which this invention is directed is uncontrolled variations in finished film thickness which undesirably affect the finished quality of the blown film. Such uncontrolled thickness variations are nonuniformities which may undesirably affect the overall quality of those products and goods which are manufactured from the film. In particular, it has been found that one such nonuniformity known as standing gauge bands results from convective updrafts caused by heat radiating in a relatively uncontrolled manner from the extruder apparatus, notwithstanding the use of the various die oscillation systems.
One approach to reducing such nonuniformities in blown film manufacturing includes the mounting of an enclosure having a square cross-section about the extruder head and extending vertically upwardly around the bubble or extrusion output. This enclosure has provided less than satisfactory results because of air turbulence in the corners of the enclosure created by air from the air cooling ring. This turbulence results in different cooling patterns inside the enclosure. In addition, the prior art enclosure provides no barrier to the heat generated by the die and the extruder beneath the enclosure. The result is relatively hot and cold regions in the enclosure and irregular and uncontrolled air flow thereby causing variations in film thickness.